The propensity of certain marine organisms like mollusks to affix themselves to man-made underwater objects has long been a difficult and expensive problem for organizations engaged in marine operations. The most well-known aspect of the problem is the fouling of ships' hulls by marine organisms which can materially retard the progress of a vessel through water. Recently, a more celebrated aspect of the problem has occurred with the invasion of the Great Lakes by a non-indigenous mollusk known as the zebra mussel.
At maturity, the zebra mussel is only a few inches long but it proliferates at a tremendous rate, forming massive colonies on underwater objects. Of most immediate concern has been the way in which these colonies have collected around water intake pipes for hydro-electric projects and municipalities, retarding the flow of water and even threatening to block it.
So far the only proven method for controlling the growth of the mussels at water intakes has been the use of chemicals, particularly chlorination. This, however, has proved to be clumsy, expensive and of some potential hazard to the underwater environment.
Also, on the Great Lakes there has been growing concern, resulting in the introduction of controls, about the use of chemicals in anti-fouling paints for the hulls of vessels. It has been concluded that some of the most effective chemicals used in these paints may be having an adverse effect on the marine environment and water quality.
Much thought, therefore, has been given to the development of a practical means of generating acoustical pulses to kill or control unwanted aquatic animals like zebra mussels in specifiable or highly localized areas. It is well known that the acoustical shock wave of an underwater explosion can locally kill or stun aquatic animals. An acoustical pulse, generated by similar or other mechanical or electrical means, can have the same local effect.
Finding a practical and controllable method of generating acoustical energy that will adversely affect unwanted aquatic organisms has been difficult. The use of explosive or mechanical underwater acoustical generators has foundered due to the inability of researchers working with these devices to solve one or several problems involving repeatability, controllability, cost, complexity, bulkiness, efficiency and general effectiveness. For instance, the one-inch air gun which is used to generate acoustical pulses requires a floating platform equivalent to a tug. This makes it vulnerable to the hazards of weather and the sea while limiting how near it can be brought to the underwater target area. The pulses it produces are also spread over a broad band of frequencies thereby limiting the energy available to those frequencies that may be found to have the optimum deleterious effect on the target organisms.
Electrically operated underwater acoustical generators, however, have existed for many years. They were developed to provide variable sound sources for the seismic exploration of bodies of water and their underlying sediments. One category of these devices involves the creating of an electric arc between two electrodes which, in effect, closely resembles a tiny explosion of TNT which breaks down the gas or water at the electrodes (creating a "plasma") and generates an acoustical pulse. Known as sparkers or plasma guns, early versions of these devices have required high voltages when submerged to ionize the water and create the arc, or mechanical techniques to provide a conductive passage between the electrodes.
Recent improvements in plasma gun design have made it a much more efficient and practical source of acoustical energy. Canadian Patent 1,268,851 by Reginald Clements et al involves feeding gas to the cavity where the arc is to occur and a means whereby the cavity can be enlarged or diminished to control the size of the plasma plume created at discharge. This makes it possible to control the wavelength of the acoustic pulse generated by the spark which is also controlled by the voltage and current supplied to the electrodes.
A plasma gun can be likened to an automobile spark plug in which the nature of the spark can be controlled by the separation of the electrodes and the amount of current and voltage that is supplied to them. In an underwater application, water may be expelled from around the electrodes and replaced by a gas before the arc or spark is created. Alternatively, sufficiently high voltage may be applied to the electrodes from a separate trigger circuit to overcome the high breakdown voltage of fresh or salt water. And like a spark plug, a plasma gun can be fired repeatedly at a high rate, e.g. one pulse per second.
The plasma gun is attractive for underwater acoustical imaging or seismic exploration because it can be operated at will over a broad range of acoustical frequencies with pulses on far narrower band widths than available from mechanical acoustical generators like air guns. It can also generate acoustical pulses at high amplitudes and specific wavelengths. This has many advantages in terms of acoustical and seismic imaging.